When film formation by vacuum deposition is repeatedly performed for a long period of time, it is important to supply an evaporated material and constantly hold a height of a liquid surface.
For example, in the case in which a material is evaporated using a 270° deflection type electron beam evaporation source, since an electron beam is radiated at an inclination with respect to a liquid surface, when the material is reduced by evaporation to decrease the height of the liquid surface (an evaporation surface), the radiation position of the electron beam becomes different from a position when the liquid surface is at its original height. Accordingly, a variation in film thickness distribution may occur.
In addition, when the material is reduced by evaporation using a resistance heating type crucible and the height of the liquid surface is lowered, a positional relation between a heating portion of the crucible and the material varies, and a variation in film thickness distribution may occur.
As methods of measuring the height of the liquid surface or an amount of the material in order to maintain a constant height of the liquid surface, a method of measuring a weight of a crucible in which a material is filled (for example, Patent Literature 1), a method using an optical imaging element (for example, Patent Literature 2), and a method of floating a float on a liquid surface (for example, Patent Literature 3) are known.